With the development of a photonic network or an optical switching technology, demand has increased for optical devices, for example, an optical switch using an optical waveguide, an optical waveguide device such as an optical modulator and so on tends to be often used.
For example, an optical switch includes a first type using a directional coupler or a Mach-Zehnder interference waveguide and a second type using a branch/cross waveguide.
One example of the optical switch of the first type operates as an analog device wherein, as a voltage applied for the switching operation of the optical switch is increased, the intensity of the output light from the optical switch gradually reduces to around zero. Then as the voltage increases furthermore, the intensity of the output light increases again. On the other hand, the second type of optical switch operates as a digital device in general, and performs on-off operation by applying a voltage not less than a certain value.
The first-type of optical switch performs an on-off operation by changing the phase of an optical wave in a waveguide. Therefore, for example, as the first type of the optical switch can perform the switching operation by only slightly changing the refractive index of a waveguide with an electro-optic effect, the first type of the optical switch has an advantage where the operating voltage is analog and also has a comparatively high operation efficiency.
However, the first-type of optical switch has a disadvantage of a comparatively large size. Also, in order to perform the on-off operation, it is necessary to set the operating voltage of the first type of the optical switch at an appropriate value according to the switching characteristic of the first type of the optical switch. As a result, an electronic circuit for controlling the optical switch has a complex construction, particularly, in the case of a device having a plurality of switch elements such as a matrix switch, it is necessary to perform a fine adjustment of the voltage to each switch.
On the other hand, the second-type of the optical switch has an advantage of having a comparatively small size. As it is enough to apply a voltage not less than a certain value in order to perform the switching operation, the second-type of the optical switch has an advantage of omitting an electric circuit for voltage control and is easier to control compared with the first-type optical switch.
FIG. 1A is a top view of such a first type of the optical switch, and FIG. 1B is a sectional view taken along line I—I of FIG. 1A. This optical device comprises a core layer 1 with a branch waveguide, a clad layer 2, electrodes 3 and 4 arranged over a branched waveguides and a substrate 5 having a heat-sink function, with the clad layer 2 being deposited thereon.
A method of utilizing a thermo-optic (TO) effect of a waveguide material raises the temperature of a waveguide directly under the electrodes 3 and/or 4 by making a proper current flow in the electrodes 3 and/or 4 and thereby changing the refractive index of the waveguide directly under the electrodes 3 and/or 4.
For example, quartz has a property that its refractive index becomes larger with the rise of temperature, namely, a positive refractive index variation coefficient, and can be used as a material for forming the core layer 1 and the clad layer 2. Raising the temperature of a waveguide directly under the electrode 3 or 4 by making a proper current flow in the electrode 3 or 4 makes an input light P0 proceed straight. Keeping the temperature of the waveguide directly under the electrode 3 or 4 comparatively low by making no current flow in the electrode 3 or 4, totally reflects the input light P0. Due to such an operation, the input light P0 to a waveguide is made into an output light P1 or P2 by a straight advance through a waveguide directly under one of the electrodes 3 and 4 or by a total reflection on the face directly under the other of the electrodes 3 and 4.
In the case of changing the refractive index of a waveguide by means of a thermo-optic effect, however, the amount of variation in refractive index is in proportion to a temperature distribution in which an electrode 6 becomes the highest in temperature, as shown in FIG. 2. Accordingly, since a total reflection face of light formed by changing the refractive index of a waveguide has the same form as the form of this temperature distribution and is not perpendicular but inclined to the surface of a clad layer 7, a part of the light reflected by the total reflection face is emitted from the waveguide and therefore the loss of light increases. Such a part of light may be coupled to the optical output waveguide being opposite to the optical input waveguide and thereby the device may be worse in crosstalk.
Also in the case of changing the refractive index of a waveguide by means of an electro-optic effect or in the case of changing the refractive index of a waveguide by means of a plasma effect of semiconductor, namely, a refractive index variation phenomenon caused by a current injection, since the distribution of electric field intensity or the distribution of electric current density is inclined to the surface of the clad layer 7, the loss of light increases.
And in such an optical device, it is preferable to improve the switching characteristic or extinction ratio of the optical device.
An object of the present invention is to provide an optical device which reduces the loss of light and crosstalk.
Another object of the present invention is to provide an optical device which improves the switching characteristic of the optical switch.
Another object of the present invention is to provide an optical device which improves the extinction ratio of the optical device.